Brazed aluminum equipment is subject to the severe problem of intergranular corrosion on surfaces coated with the brazing alloy. This is caused by migration of silicon-rich material from the brazing alloy layer into the parent metal during the brazing process. The more noble potential of conventional brazing alloy claddings with respect to the core alloys galvanically accelerates intergranular corrosion. As a consequence, failures due to complete penetration or mechanical weakening can occur. It has been observed that brazed headers, fins and tubes in automotive radiators fabricated from, for example, vacuum brazing Alloy 4004 (an aluminum base alloy containing about 9.5% silicon, 1.5% magnesium, up to 0.3% iron, up to 0.05% copper, up to 0.07% manganese, up to 0.01% titanium, and the balance essentially aluminum) clad onto aluminum Alloy 3003 (an aluminum base alloy containing from 1.0 to 1.5% manganese, from 0.05 to 0.20% copper, up to 0.7% iron, up to 0.6% silicon, up to 0.1% zinc, balance essentially aluminum) designated by the Aluminum Association as No. 7 or 8 brazing sheet, are particularly susceptible to this form of damage. In addition, gas liquefaction equipment and heat exchangers generally also suffer from intergranular corrosion.
The corrosive environments which can cause this problem include water containing dissolved chloride, bicarbonate or sulfate ions, especially if the pH of the water has a relatively low value. Such waters may condense as films on the fins of heat exchanger equipment used for automotive or aircraft air conditioners, automotive radiators, gas liquefaction equipment or the like.
Intergranular corrosion has also been encountered in other applications, as on brazed headers inside automotive radiators and heat exchangers generally. In such cases, the coolant is usually corrosive. For example, if automotive antifreeze solutions are used, poor maintenance can often result in the solution becoming corrosive for a variety of reasons. Chief among these reasons is that the antifreeze may have been allowed to remain in the radiator for a number of years without replacement while replenishing the level with mixtures of fresh antifreeze solution or hard natural water. These practices would deplete the corrosion inhibitors and reserve alkalinity components, permitting the coolant pH to drop and allowing heavy metal ions to accumulate from reaction of the acids with copper alloy and cast iron surfaces in the coolant system.
U.S. Pat. Nos. 3,898,053 and 3,853,547 describe certain aluminum-silicon brazing compositions for joining aluminum alloy components; however, these compositions do not solve the problem of intergranular corrosion described hereinabove.
Accordingly, it is a principal object of the present invention to provide an improved brazed aluminum composite which is characterized by substantial resistance to intergranular corrosion.
It is a still further object of the present invention to provide a composite as aforesaid which is inexpensive and convenient to use on a commercial scale.
Further objects and advantages of the present invention will appear hereinbelow.